The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the line pressure in a hydraulic circuit for the transmission.
A known control system for a continuously variable belt-drive transmission disclosed in U.S. Pat. No. 4,369,675 comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the tranmission ratio in accordance with the opening degree of a the throttle valve of the an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio, the engine speed and driving conditions. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine and to enable the system to change the transmission ratio at proper speeds.
At idling of the engine, the transmission ratio is set at a maximum value, and the line pressure is at a maximum value. When the accelerator pedal of the vehicle is depressed, a clutch is engaged to start the vehicle. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshift). At that time if the engine speed is kept constant, the tranmission ratio is automatically and continuously reduced at a speed which is determined by the line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio.
In such system, it is preferable to control the position of the spool of the line pressure control valve by a control oil which is supplied to an end of the spool through a control valve. The position of the spool is controlled by controlling the quantity of oil draining from the control valve. A line pressure control system is proposed, in which at a maximum quantity of the drain oil, the line pressure is kept at a maximum value. That is, at idling of the engine, the line pressure is at a maximum value and the quantity of the drain oil is maximum. However, at idling of the engine, the discharge of the pump is at a minimum. Accordingly, problems such as reduction of the line pressure in the hydraulic circuit may arise at idling of the engine. In order to resolve such a problem, a pump having a large capacity must be provided, which will cause an increase of oil pump loss in the transmission.